In designing high-speed communications system devices, such as high-speed optoelectronic transceivers, signal integrity is a paramount issue. With extremely fast signals and corresponding short bit periods, such as those utilized in a 10 Gb/s optoelectronic transceiver, even slight degradation of the signal can render the system unuseable. This is problematic because even relatively small transmission-line discontinuities can lead to significant signal degradation in a high-speed communications system.
One particularly problematic location for transmission line discontinuities in a high-speed communication system is the interface between two different electronic components, such as the interface between a plug-in module having an edge-card connector system and the edge-card connector on a host printed circuit board (PCB), or the interface between a surface-mounted chip and the PCB on which the chip is mounted. These types of interfaces tend to introduce discontinuities in the transmission path of the communication signals.
For example, an edge-card connector system typically uses a series of spaced-apart contact pads on a plug-in card or module that engage and electrically couple to a series of spaced-apart complementary connectors in a connector unit on a separate PCB. An example of a connector unit is shown in FIG. 1, and an example of a plug-in card or module is shown in FIG. 2. This type of connection can be used with, for example, an optoelectronic transceiver that plugs into a host PCB. The edge-card connection mechanism, well-known in the art, makes mounting electronic cards and devices onto a host PCB solder-less and easy. It also tends to introduce discontinuities in the signals that pass through the complementary connectors into the contact pads and onto the card or module containing the contact pads. These discontinuities are likely caused by several features of the physical design of such a system.
In particular, some of the physical features used by an edge-card connector unit for structural support and connectivity purposes likely cause discontinuities in the signal transmission path. As illustrated in FIG. 1, external electrical device connector 100, also referred to as a connector unit, is typically comprised of a series of spaced-apart complementary connectors, also referred to herein as contact points. (In the side plan view of FIG. 1, only one complementary connector 102 is depicted, however). Each complementary connector has a connector-arm portion (i.e., the top-most curving part of the complementary connector) that electrically couples with a contact pad on a card or module having an edge-card connector system. To mount complementary connector 102 in a perpendicular fashion to host PCB 110, the complementary connector must lock against connector body 108. This is typically accomplished using a mounting feature 106, which is shown in FIG. 1 as an integral mounting pin that fits into a mounting hole on the connector body. Other types of structural interlocking mechanisms in addition to a pin and hole pair are also commonly used.
The purpose of locking complementary connector 102 to connector body 108 is to provide additional structural stability for the complementary connector 102. However, this design also likely adds undesirable electrical effects into the transmission path of electric signals conducting through the complementary connector. Because complementary connector 102 is a conductor, mounting feature 106 is also typically a conductor, due to manufacturing requirements. Thus, mounting feature 106 adds increased planar surface area to the transmission path. This, in effect, adds excess shunt capacitance to the transmission path, relative to the nominal capacitance of the path. (A transmission path has a nominal impedance per unit length—e.g., 50 ohms in a single-ended transmission path—and, relatedly, a nominal series inductance and shunt capacitance per unit length). This excess shunt capacitance is also referred to as parasitic capacitance. This undesirable capacitance results from interaction with other portions of the path, such as with the connector arm portion of complementary connector 102, and with the grounding plane (not shown in FIG. 1).
Another possible source of unwanted discontinuities is the connector arm portion of complementary connector 102. When a plug-in card or module is slipped into the opening shown on the right side of external electrical device connector 100, the contact pads on the top of the card will couple with the complementary connector 102, and the contact pads on the bottom of the card will couple with the bottom-edge connector contacts 104 (typically used for low-speed communications). To ensure that complementary connector 102 reliably and easily mates with a contact pad, the connector arm portion presents a convex surface relative to the contact pad. Using this particular design, the contact pads of a plug-in card or module can easily be slid along the connector arm portion of the complementary connector 102, providing an electrical coupling without impeding the movement of the plug-in card or module. Depending on the angle of the complementary connector 102 relative to the plane of the plug-in card, the complementary connector can also provide additional support and resistance against extraneous movement by the plug-in card.
However, this layout also causes part of the complementary conductor, and hence the transmission path, to extend past, and up and away from, the actual point of contact with a contact pad (i.e., because of the “hooked” end). Like with the mounting feature 106, this causes parasitic shunt capacitance to appear in the transmission path in excess of the nominal shunt capacitance associated with the transmission path.
Referring to FIG. 2, a typical edge-card connector 200 is shown. Printed on a circuit board are a series of spaced-apart contact pads. As shown in FIG. 2, the contact pads may include ground contact pads 202-1, 202-2 . . . 202-n, signal contact pads 204-1, 204-2 . . . 204-n, and power contact pads 208-1, 208-2. The ground contact pads are typically longer than the signal contact pads to allow for hot-plugging of the edge-card connector, by ensuring that the system is grounded before power and signal connections are made. The ground contact pads generally connect to a trace extending a short distance beyond the ground contact pad, which then connects to a connector going downward to a (typically) internal ground plane, separated from the top surface by a dielectric material.
The signal contact pads of the edge-card connector connect to a signal line, such as signal lines 206-1, 206-2 . . . 206-n. The signal contact pads are typically wider than the signal wires. The purpose of having contact pads with a wide surface area is to facilitate an easy coupling with a complementary connector, because the connector need only contact any portion of the wide surface area to establish an electrical connection. However, it is likely that this extra surface area also increases the shunt capacitance between each contact pad and the ground plane, as well as between the contact pads themselves, resulting in excess, parasitic shunt capacitance that causes the aforementioned transmission-line discontinuities.
Edge-card connector 200 will also typically include at least one contact pad for power, such as power contact pads 208-1 and 208-2, at a predetermined voltage level. The length of the power contact pads tends to be intermediate the length of the ground contact pads and the length of the signal contact pads, again to allow for hot-plugging by ensuring that the edge-card connector system is powered before signaling connections are made (but only after grounding has been achieved). Of course, the card containing the edge-card connector will also include various electrical components, such as resistors, capacitors, potentiometers, inductors, diodes, etc. These components will typically be interconnected through various traces and signal lines, and are typically utilized in the transmission path as well. The card may also contain, or comprise, other specialized components, such as optoelectronic components, processors, ASICs, and the like.
Regardless of the purpose of the plug-in card or module on which an electrical connection system is mounted, it would be desirable to provide an electrical connection system that reduces discontinuities in the transmission path through the interface between the plug-in card and the host PCB.